Abstract

Although there are several therapies available for the treatment of breast cancer, many patients have tumor recurrences that ultimately metastasize. There are several oncogenic estrogen receptor alpha (ERα) coregulators that promote breast cancer metastasis and targeting these coregulators could be a promising cancer therapeutic. One coactivator of ERα that is known to be upregulated in breast cancer and promote metastasis is proline-, glutamic acid-, and leucine-rich protein 1 (PELP1). PELP1 affects the migratory and invasive potential of breast cancer cells both in vitro and in vivo and modulates the expression of several genes involved in the epithelial to mesenchymal transition (EMT) including Snail, Twist and Zeb1. To elucidate the mechanism by which PELP1 promotes metastasis, we studied the proteins that interact with PELP1 and promote cancer metastasis. We identified a novel PELP1-interacting protein G9a (Bat8/KMT1C/EHMT2), a histone lysine methyltransferase known to overexpressed in aggressive breast cancer and promote invasion and metastasis. G9a contributes to the epigenetic silencing of tumor suppressor genes, interacts with Snail to promote EMT and its knockdown inhibits cell migration and invasion as well as lung metastasis. The objective of this study is to characterize the interaction of PELP1 and G9a in the promotion of breast cancer metastasis. The complex formation of PELP1 and G9a was confirmed by an in vivo immunoprecipitation and an in vitro binding assay. The region of PELP1 that binds to G9a was mapped further by PELP-GST deletions. To determine whether formation of the complex is necessary for PELP1's oncogenic functions including the promotion of metastasis, we developed a peptide to disrupt the complex formation. The sequence of the peptide has homology to both PELP1 and G9a with a TAT-sequence to enable cellular uptake. Cellular uptake was confirmed by fluorescent microscopy while biotin-avidin peptide pull-down assays confirmed PELP1-peptide binding. A peptide competition assay confirmed that the peptide could interrupt PELP1-G9a interaction. Treatment of two ER-positive, PELP1-overexpressing breast cancer cells (MCF7-PELP and ZR75-PELP) with the peptide significantly inhibited the PELP-1 mediated proliferation. Peptide treatment also substantially inhibited PELP1-mediated cell migration in Boyden chamber assays as well as decreased anchorage independence as seen by soft agar assay. Interestingly, disrupting PELP1-G9a interaction with the peptide also sensitized tamoxifen and letrozole resistant cells to the respective hormonal therapy. Mechanistic studies revealed that PELP1-G9a interactions play a critical role in epigenetic changes in therapy resistant cells. In conclusion, our studies identified a novel complex involved in breast cancer metastasis. The interaction of PELP1 and G9a is required for PELP1-mediated oncogenic properties and disruption of the complex by peptide treatment results in decreased metastatic potential and changes in epigenetic modifications. The novel peptide could be used for therapeutic targeting of breast cancer metastasis and therapy resistance. This work was supported by the NIH grant CA095681 and CPRIT pre-doctoral fellowship RP101491.